The present disclosed subject matter relates generally to valve assemblies, and in particular to a two stage air operated valve assembly.
Sealed containers, such as tank trailers, allow for the storage and transportation of solid and liquid materials. Vapors generated by material within a sealed container, such as vapor generated by a liquid, create pressure on the container walls and any sealed outlets communicating with the interior of the container. It is often desirable to allow the vapor to accumulate within the sealed container until the material is transferred from the container or until the vapor can be safely vented. The accumulated vapor can generate pressure within the tank making it difficult to transfer material into or out of the container. Valves connected to an outlet of the container provide a seal for preventing movement of the vapor out of the container until it is desired to transfer material from the container or vent the vapor from the container. The seal is provided by a sealing mechanism covering the outlet. However, high pressures within the container can create forces that act on the seal and exceed the force available to open the seal, thereby preventing the valve from opening and the container from venting.